Gay and bisexual men have high rates of human papillomavirus (HPV) infection and HPV-related disease (e.g., anal cancer) compared to the general population. HPV vaccination is a promising strategy for reducing these existing disparities, as routine HPV vaccination is recommended for young gay and bisexual men (YGBM) through age 26. However, little research has addressed HPV vaccination among YGBM, and no interventions we are aware of have been developed specifically for this high-risk population. To address this important research gap, we conducted a national survey of YGBM ages 18-26 in late 2013. Only about 13% of participants had received any doses of HPV vaccine, and results provided valuable information for designing an HPV vaccine intervention for YGBM. Based on results of this preliminary research, an interdisciplinary research team will conduct the proposed study, Increasing HPV Vaccine Coverage among Young Adult Gay and Bisexual Men. The study will develop and pilot test a theoretically-based mobile health (mHealth) HPV vaccine intervention for YGBM that will consist of: 1) population-targeted, individually-tailored content about HPV and HPV vaccine; and 2) vaccination reminders. During pilot testing, the intervention will be compared to a control group that receives standard information about HPV and HPV vaccine via a mobile-friendly website. The study will have two primary aims and an exploratory aim. Aim 1 will develop and pilot test the mHealth HPV vaccine intervention for YGBM to establish intervention feasibility and acceptability. Feasibility will be established by meeting recruitment (n=94) and retention (85% retention over a 6-month follow-up period) goals for the pilot study, and acceptability will be established by comparing participant satisfaction with study materials between the intervention and control groups. Aim 2 will obtain preliminary efficacy data on whether the mHealth HPV vaccine intervention increases HPV vaccine initiation compared to the control group. HPV vaccine initiation (i.e., receipt of the first dose of the three-dose HPV vaccine series) will serve as the primary outcome for the pilot study. An exploratory aim will examine whether the mHealth HPV vaccine intervention affects secondary outcomes compared to the control group. Secondary outcomes will include receipt of the second and third doses of the HPV vaccine series and changes in potential mediators between study group and HPV vaccination. Results from the pilot study will establish intervention feasibility and acceptability and provide valuable preliminary data on outcomes related to HPV vaccination. These data will inform an R01 grant application to test the efficacy of the mHealth HPV vaccine intervention in a large randomized controlled trial.